This invention relates to a fluid pressure energy translating device and more particularly to an improved high pressure hydraulic axial piston pump or motor.
Axial piston pumps or motors generally comprise an annular block or barrel defining a plurality of cylinders and aligned passages arranged concentrically about a barrel axis which each slidably receives one of a like plurality of pistons. The pistons include spherical bearings received within shoes which operably couple them to a stationary wobble plate or cam plate disposed adjacent one end of the barrel. The cam plate may be secured at a fixed angle relative to the barrel axis or may be disposed in trunnions such that the angle between the cam plate and barrel axis may be varied. The shoes slide on the cam plate as the barrel is rotated. Reciprocation of the pistons in response to relative rotation between the cam plate and the barrel is thus effected. The barrel is supported on a drive shaft for rotation about its axis and abuts a fixed port plate which engages the end of the barrel opposite the cam plate. The port plate has a pair of ports or passages which provide independent communication between a source of fluid and a discharge line. The ports in the port plate register with the plurality of passages in the barrel which communicate with the individual cylinders so that fluid will be alternately introduced into and discharged from each cylinder as the barrel is rotated and the pistons reciprocate.
In recent years, hydraulic component applications in various industries have become increasingly taxing. For example, axial piston pumps and motors are being asked to far exceed their design capabilities. Increases in both hydraulic pressure and rotational speeds are causing higher rates of failure in axial piston pumps and motors. Failures primarily occur in the form of barrel-port plate separation resulting in a loss in pressure or "blow-off" and loss of shoe contact with the surface of the cam plate.
In order to maintain a fluid seal between the rotating barrel and the fixed port plate under all operating conditions, several requirements must be met. First of all, the mating surfaces must be extremely flat and perfectly parallel. Typically, the mating surfaces are flat to within two lightbands of flatness. Furthermore, proper axial alignment between the barrel and the port plate must be maintained. If the barrel and port plate slightly axially misalign, i.e., tilt relative to one another, increased wear of the mating surfaces on the port plate and the barrel will occur. If the tilt is great enough, system pressure will act on usually unexposed area causing barrel-port plate separation or "blowoff."
Finally, while such a fluid seal is assisted by fluid pressure exerted on the barrel when the device has reached operating speed, it is necessary to provide supplemental biasing means to ensure intimate barrel to port plate contact when the pump or motor is just beginning to rotate or when the variable cam plate is positioned perpendicularly to the barrel axis such that zero flow and zero pressure exists in, for example, an electro-hydraulic servo circuit used to provide cross center (reverse flow). A conventional coil spring has been utilized in prior art designs to provide such bias. The constant force versus deflection relationship of a coil spring as well as the physical placement of the spring generally between the barrel and the drive shaft creates certain assembly and operational difficulties. With regard to the former, since a specific barrel-to-port plate bias is required it must be achieved by compressing the spring an appropriate amount. Variations in spring free length, spring rate, coil end finish, etc., necessitate repeated adjustment, assembly and force measurement steps in order to achieve a desired biasing force in each production unit. With regard to the latter, as the abutting surfaces of the barrel and port plate wear during use, the biasing force will drop in accordance with the spring rate relationship. Since springs suitable for this application exhibit a relatively high spring rate, even a small quantum of wear will result in an appreciable reduction of biasing force and this increased likelihood of improper operation, particularly at low pressures and speeds.
Barrel-port plate contact or alignment is a complex problem since the forces acting upon these components and thus urging them out of alignment are both numerous and dynamic. As noted above, fluid pressurization and pumping by the pistons is accomplished by interaction between the cam plate and piston shoes. The force which the cam plate exerts on each of the piston shoes to pump fluid is balanced by a reaction force in the opposite direction. Due to the inclination of the cam plate, however, this axial reaction force produces a radial component of force tending to move the piston shoes radially away from the barrel axis. The forces from each of the piston shoes may be resolved into a single resultant force acting on the barrel and extending radially from the barrel axis at the point of intersection of the barrel axis and the plane of loci of the centers of the spherical bearings. The magnitude of this force is proportional to the hydraulic fluid pressure. It is therefore dynamic but independent of the rotational speed of the barrel.
A second force tending to tilt the barrel results from centrifugal force. The centers of gravity of certain of the pistons are axially offset from those of diametrically opposed pistons. The centrifugal force on each piston acts through the center of gravity of the piston in a radial direction. Since the centers of gravity of some of the pistons are axially offset from others, an unbalanced centrifugal force is applied to the barrel. The centrifugal force on diametrically opposed offset pistons applies a dynamic couple to the barrel which is the product of the centrifugal force acting on one of the pistons times the axial offset of the centers of gravity of the pistons. The magnitude of this couple will vary from zero in the case of opposed pistons in which the centers of gravity are aligned at right angles to the shaft to a maximum value in the case of opposed pistons in their maximum offset position. The magnitude of the couple is also directly related to speed.
A general solution to these problems which has been incorporated into the design of most contemporary axial piston pumps comprehends restraining either the barrel or port plate while permitting the other a certain amount of orientation freedom. Through this approach, barrel-port plate misalignment which might result in leakage and blow-off is minimized since tilting or skewing of one of the elements may be accommodated by movement of the other. Another approach is disclosed in my prior U.S. Pat. No. 3,126,835. By supporting the barrel on a bearing on a drive shaft extending coaxially through the barrel and by properly locating the bearing, the effects of the dynamic couple resulting from centrifugal force may be offset at least in part by the effects of the resultant force from fluid pressure acting on the pistons.
The unequal forces acting on the barrel also tend to slightly deflect the drive shaft which supports the barrel. If the barrel is rigidly connected to the drive shaft and the drive shaft deflects or bends slightly under loading, the barrel will tilt relative to the port plate and a loss of fluid pressure will occur. In my prior U.S. Pat. Nos. 3,126,835 and 3,160,109, a loss of fluid pressure resulting from deflection or bending of the drive shaft is reduced through the use of a torque tube interconnecting the drive shaft with the barrel and through the use of a crowned bearing between the drive shaft and the barrel. The torque tube extends coaxially along the drive shaft between the drive shaft and the barrel and has one end connected through splines to the drive shaft and an opposite end connected through splines to the barrel. As the shaft is driven, the torque tube in turn drives the barrel. The splines between the drive shaft and the torque tube and between the torque tube and the barrel also may be crowned to allow the shaft to flex relative to the barrel without tilting the barrel, as taught in my U.S. Pat. No. 4,232,587. As the drive shaft flexes under loading, the barrel is permitted to slide on the port plate without tilting away from the port plate. This construction has been effective in greatly reducing or eliminating tilting of the barrel and the resulting hydraulic fluid leakage.